This invention relates to a process for manufacturing fluoropolymers. More particularly, the present invention relates to using imide salts as emulsifiers for polymerizing fluoroolefins.
Fluorine-containing polymers, or fluoropolymers, are used in many commercial products. Fluoropolymers are known to exhibit chemical inertness (i.e., resistance to chemical attack), high thermal stability, usefulness at high temperatures, and toughness and flexibility at low temperatures.
Useful fluoropolymers include both fluoroelastomers and fluoroplastics. Fluoropolymers can contain interpolymerized units derived from olefinically unsaturated monomers, such as monomers of vinylidene fluoride, hexafluoropropylene, tetrafluoroethylene, etc. Fluoroelastomers (e.g., FLUOREL(trademark) (available from Dyneon, LLC, St. Paul, Minn.) and VITON(trademark) (available from E. I. DuPont de Nemours and Company, Wilmington, Del.)) have particular utility in high temperature applications, such as seal gaskets and linings. Fluoroplastics (e.g., THV(trademark) 200, THV(trademark) 400, THV(trademark) 500G, THV(trademark) 610X available from Dyneon LLC), particularly those containing interpolymerized monomeric units derived from chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride, have numerous electrical, mechanical, and chemical applications. As an example, these fluoroplastics are useful in wire, electrical components, seals, solid and lined pipes, and pyroelectric detectors. Examples of fluoropolymers include, but are not limited to, poly(tetrafluoroethylene) (PTFE), perfluoroalkoxyalkane (PFA), perfluoro(ethylene-propene) copolymer (FEP), ethylene-perfluoroethylenepropene copolymer (EFEP), tetrafluoroethylene-perfluoro(dioxole) copolymer (TFE/PDD), vinylidene fluoride-tetrafluoroethylene copolymer (VDF/TFE), polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), polychlorotrifluoroethylene (PCTFE), and ethylene-chlorotrifluoroethylene copolymer (ECTFE).
One method of obtaining fluoropolymers is by emulsion polymerization of the fluorinated monomers in aqueous media. In emulsion polymerization, the emulsifier is normally used at a high enough concentration such that micelles, which serve as the loci for polymerization, are present in the aqueous phase. The emulsifier also provides colloidal stability to the formed polymeric particles through electrostatic repulsion and/or steric stabilization to prevent particle agglomeration and coagulation.
Perfluorinated surfactants have been used as emulsifiers in the emulsion polymerization of fluoroolefins to minimize chain transfer. They allow the polymerizations to be conducted in aqueous media, and also improve the physical properties of the fluoropolymer and increase the rate of polymerization. A perfluorooctanoyl fluoride (PFOF) derived emulsifier, ammonium perfluorooctanoate, has been the preferred emulsifier for commercial production of fluoropolymers. It has been reported that certain perfluorooctyl-containing compounds may tend to have low bioelimination rates in living organisms as well as persistence in the environment. This tendency has been cited as a potential concern for some fluorochemical compounds. See, for example, U.S. Pat. No. 5,688,884 (Baker et al.). Introduction of an emulsifier and its degradation products into the environment may occur either from streams related to the use of the emulsifier itself or to waste streams/emissions from its manufacture.
As a result, there is a desire for fluorine-containing compositions that are effective in providing desired emulsifier properties, that eliminate more rapidly from the body, that degrade in the environment to non-bioaccumulative degradation products, and that is not prone to accumulation in living organisms. The precursor to these emulsifiers should preferably be less bioaccumulative than PFOF derived materials to prevent the possible introduction of bioaccumulative and persistent materials into the environment from manufacturing streams. In addition, the emulsifiers preferably can be manufactured in a cost-effective manner.
The present invention provides imide salts useful as emulsifiers for the polymerization of fluoroolefins. Advantageously, the emulsifiers of the present invention are comprised of imide anions that have good surface activity while being physically and chemically stable during their use as emulsifiers. The emulsifiers of the present invention are expected to gradually degrade in the environment and will likely more rapidly bioeliminate than ammonium perfluorooctanoate. In addition, the emulsifiers of the present invention can be produced from lower cost intermediates, such as C4F9COF and C4F9SO2F, compared with emulsifiers derived from C7F15COF (PFOF), which is produced in relatively low yields by electrochemical fluorination (ECF).
The present invention comprises a method of using imide salts as emulsifiers to emulsion polymerize fluoroolefins.
The present invention comprises a method for preparing fluoropolymers comprising emulsion polymerizing in an aqueous phase, under free-radical conditions, at least one fluoromonomer in the presence of at least one emulsifier and at least one initiator, wherein said emulsifier comprises at least one anion of the following formula: 
where each Q is independently CO or SO2 and each Rf is independently a perfluorinated alkyl group; and a non-interfering cation.
Another embodiment of the present invention is a method wherein the aqueous phase further comprises one or more chain transfer agents.
Yet another embodiment of the present invention is a method wherein the aqueous phase further comprises at least one non-fluorinated monomer.